Regulation of bitterness in cucumber

Sanwen Huang and colleagues report the identification of two transcription factors necessary for bitterness in the fruit and leaves of cucumber (Science 346, 1084–1088, 2014). The previously identified loci Bitter (Bi) and Bitter fruit (Bt) control bitterness, which is mainly due to cucurbitacin C (CuC), in cucumber and related plants. The authors performed genome-wide association analysis for variants associated with Bi using the cucumber genome variation map. They identified Csa6G088690, encoding an oxidosqualene cyclase, as the cucumber Bi gene. To further elucidate the mechanism for CuC biosynthesis, the authors screened a cucumber ethyl methanesulfonate (EMS) library for plants with non-bitter leaves. They sequenced the genomes of two mutants and identified a SNP in Csa5G156220, which encodes a transcription factor expressed exclusively in leaves. They found that this gene, renamed Bitter leaf (Bl), regulates the expression of Bi. Eleven variants in a nearby homolog of Bl (Csa5G157230) associated with extreme bitterness in a local association analysis, and one variant cosegregated with Bt in an F₂ population, indicating that it is likely the Bt gene. The authors confirmed that Bt regulates Bi expression in fruits. Finally, the authors identified nine genes downstream of Bl and Bt in the CuC biosynthetic pathway through a candidate gene RNA interference screen.